PRBOBLEM WITH MK IV COMBO

doobchec · Sep 17, 2009

I've recently experienced a loss of volume when playing through various channels on my Mk IV combo. Switching channels doesn't help either. I have a changed out a couple of suspect 1/4" chords and thought that did it. But the issue came up again. It has been a long while sinse any of the tubes were replaced with new ones. I know tubes may be the issue....but I have checked the tubes and I don't notice anything obviously wrong. Can anyone offer any insight or should I throw down $ for a re-tube package? Thanks for the help!

dodger916 · Sep 17, 2009

If it's just a loss of volume across all channels WITHOUT static, swishing or microphonics, it's probably power tubes. If one of the aforementioned noises are present, it could be a preamp tube and/or power tubes. If you haven't changed them in a REAL long while, perhaps it's time for a re-tube. I'd change power tubes first as preamp tubes have a much longer service life. Plus which, if you play the amp with any regularity, you WILL need new power tubes eventually. It's also good to keep a spare pair or two on hand. If you still have problems, it might be preamp tubes, or possibly something else.

You can perform a quick check of power tubes which might isolate any weak tube(s). I'd try running the amp in Simul, then switch to Class A. If the volume is restored in Class A, I'd replace the inner pair of tubes. If no volume change when switching to Class A, then I'd swap the inner and outer PAIRS of tubes (with power off!) and re-run the experiment. If volume is restored in Class A, then replace the now-inner pair. If no change, then you might need to replace them all.

Again, it might not be tubes.

GD_NC · Sep 17, 2009

doobchec said:
I've recently experienced a loss of volume when playing through various channels on my Mk IV combo. Switching channels doesn't help either. I have a changed out a couple of suspect 1/4" chords and thought that did it. But the issue came up again. It has been a long while sinse any of the tubes were replaced with new ones. I know tubes may be the issue....but I have checked the tubes and I don't notice anything obviously wrong. Can anyone offer any insight or should I throw down $ for a re-tube package? Thanks for the help!

I'd try new power tubes. If you haven't changed them in a while it's probably time anyway. While you are at it, order a new preamp tube tube. If the power tubes don't solve the problem you can swap the new preamp tube one by one through the pre-amp positions. If that does not work, it's probably time to take it to your local tech.

One other thought: Put your Mark IV in tweed/class A (using only the outside pair of tubes) and see if the problem improves. If not, (Assuming you have 6L6 tubes in all 4 positions!) you could swap the inter and outer pair of tubes. That
might reveal the bad power tube.

Edit: well it looks like Dodger beat me to the answer. At least we are consistent.

primeholy · Sep 17, 2009

Before you replace tubes, try cleaning your effects loop. This happened to my .22(that I dont have anymore

) Or if you change your tubes and it still happens clean your loop.

doobchec · Sep 17, 2009

I tried cleaning the loops and inputs and that hasn't done the trick. I'm guess I'll change the power tubes first and go from there.

primeholy · Sep 17, 2009

doobchec said:
I tried cleaning the loops and inputs and that hasn't done the trick. I'm guess I'll change the power tubes first and go from there.

Good luck! Hopefully it is just the tubes! Do you have new tubes yet? I have heard good things about winged c tubes.

doobchec · Sep 17, 2009

I have not purchaced new tubes yet...although I think I've narrowed my decision down to a quad of JJ 6L6/5881 or Mesa's version of the same. I've never tried JJ's. I've had Mesa 6L6's for a while. But I was looking to change things up a bit. I haven't heard of "winged c tubes."